A conventional optical pick-up device for magneto-optical disc has a configuration as shown in FIG. 22. At the time of reproduction (or at the time of recording), laser beams of so called P polarization component are emitted from a laser diode 201. These laser beams are changed into rays of parallel light by a collimator lens 201a, and are caused to undergo shaping by a shaping prism 202. The rays of light thus shaped are incident to a S polarization beam splitter 203.
A polarization beam splitter film 203a provided at the S polarization beam splitter 203 has the characteristic that it allows 100% of rays of light of S polarization component having polarization direction perpendicular to the P polarization component to be reflected, and allows 50% of rays of light of P polarization component to be reflected and allows the remaining 50% thereof to be transmitted therethrough. For this reason, one half of the laser beams of the P polarization component incident to the S polarization beam splitter 203 are reflected and the remaining half thereof are transmitted therethrough. The laser beams transmitted through the S polarization beam splitter 203 are reflected by a 45 degree mirror 204A, and are irradiated onto a magneto-optical disc 206 through an object lens (objective) 205.
In this example, at the time of recording, data is delivered to a magnetic head 208 through an input terminal 207. Thus, the magnetic head 208 is driven in accordance with the data, so a magnetic field corresponding to the data is produced. This magnetic field is applied to the portion where laser beams are irradiated of the magneto-optical disc 206. Thus, recording of data is carried out.
When laser beams are irradiated onto the magneto-optical disc 206 as described above, a reflected light is produced. Rays of this reflected light are reflected by the 45 degree mirror 204A through the object lens 205, and are incident to the S polarization beam splitter 203.
The rays of reflected light are reflected in a manner deviating with respect to the S polarization component in accordance with data recorded on the magneto-optical disc 206. However, such deviation quantity is very small, and most of such rays of light are reflected as the P polarization component. The S polarization beam splitter 203 allows 100% of rays of the S polarization component to be reflected, and allows 50% of rays of the P polarization component to be reflected and allows the remaining 50% to be transmitted therethrough as described above. For this reason, rays of reflected light of the S polarization component of the rays of the reflected light are all reflected by the S polarization beam splitter 203 and are incident to a polarization beam splitter 204. On the other hand, one half of rays of reflected light of the P polarization component are reflected and the remaining half are transmitted therethrough by the S polarization beam splitter 203.
A polarization beam splitter film 204a of the polarization beam splitter 204 has the characteristic that it allows all of rays of light of the P polarization component to be transmitted therethrough and allows all of rays of light of the S polarization component to be reflected. For this reason, rays of the reflected light of the P polarization component of the rays of reflected light incident to the polarization beam splitter 204 are transmitted through the polarization beam splitter film 204a and are incident to a servo signal detection system 213. Moreover, rays of the light of the S polarization component are reflected by the polarization beam splitter film 204a and are incident to a data detection system 214.
The rays of reflected light of P polarization component incident to the servo signal detection system 213 are converged through a lens 210 and a cylindrical lens 211, and are irradiated onto a photodetector 212 for detection of servo signal. The photodetector 212 receives the reflected light of P polarization component to deliver a light quantity detection signal corresponding to the received light quantity to servo signal detecting circuit (not shown). The servo signal detecting circuit detects, on the basis of the light quantity detection signal, a focus error and a tracking error to deliver both a signal indicative of the detected focus error and a signal indicative of the detected tracking error to servo control circuit (not shown). The servo control circuit carries out a tracking error control and a focus error control on the basis of the focus error signal and the tracking error signal. Thus, it is possible to carry out data reproduction, etc. under the state of just track and just focus (in focus) at all times.
In a manner stated above, in the S polarization beam splitter 203, 50% of rays of a light of the P polarization component are reflected and the remaining 50% are transmitted therethrough. In the servo signal detection system 213, a tracking error and a focus error are detected on the basis of a reflected light of P polarization component reflected by the S polarization beam splitter 203. In this case, since most of the rays of the reflected light are reflected as the P polarization component, even if the above-mentioned S polarization beam splitter 203 is adapted to allow 50% of rays of light of the P polarization component to be reflected and to allow the remaining 50% to be transmitted therethrough, it is possible to detect such tracking error and or focus error by a sufficient light quantity.
On the other hand, ray of reflected light of S polarization component reflected by the polarization beam splitter 204 are converted (changed) into rays of a reflected light of P polarization component by a .lambda./2 plate 206 of the data detection system 214, and are incident to a polarization beam splitter 20% A polarization beam splitter film 207a of the polarization beam splitter 207 has the characteristic that it allows 50% of rays of light the P polarization component to be reflected and to allows the remaining 50% thereof to be transmitted therethrough. For this reason, rays of the reflected light of the P polarization component incident to the polarization beam splitter 207 are divided into two (groups of) rays of light by the polarization beam splitter 207, and the rays of light thus halved are respectively irradiated onto photodetectors 208, 209 for detection of data.
The respective photodetectors 208, 209 respectively receive the above-mentioned two (groups of) rays of reflected light to deliver light quantity detection signals corresponding to the received light quantities to data detecting circuit (not shown). The data detecting circuit detects data on the basis of the respective light quantity detection signals to deliver the data thus detected to a data processing system. The data processing system implements a predetermined data processing to the data to deliver, at the time of reproduction, such processed data to external equipment such as a computer system or a speaker system, etc.
Thus, in the computer system and/or the speaker system, etc., data reproduced from the magneto-optical disc 206 is suitably processed.
However, since the above-described conventional optical pick-up device for magneto-optical disc is adapted to separate laser beams and rays of reflected light by using polarization beam splitters 203, 204, 207 to detect data, focus error and/or tracking error, etc., there result increased number of parts and increased cost. In addition, since there is a necessity of ensuring light path for rays of reflected light, etc. separated at the polarization beam splitters 203, 204, 207, the optical pick-up device itself was disadvantageously large-sized.
Moreover, in the above-mentioned polarization beam splitter 204, reflection and transmission of rays of light of the P polarization component were caused to be respectively 50%. In this case, the reflection factor is set on the basis of light quantity irradiated to the servo signal detection system 213 and shot noise of the photodetector 212 for detection of servo signal or noise resulting from double refraction, etc. of magneto-optical disc 206. The coupling efficiency and the S/N ratio have the trade-off relationship. For this reason, there is the problem that if the coupling efficiency is caused to be higher, the S/N ratio is lowered, and conversely if the S/N ratio is caused to be high, the coupling efficiency is lowered.
In addition, it is expected that optical pick-up devices capable of reproducing not only data recorded by light and magnetism like magneto-optical disc, but also data recorded as prepits like compact disc will be developed.
This invention has been made in view of such actual circumstances, and its object is to provide such an optical pick-up device to allow the polarization beam splitter to become unnecessary to permit, through reduction of the number of parts, reduction of cost of the device and realization of the device of compact structure, to permit coping (dealing) with reproduction of the magneto-optical disc and reproduction of the compact disc, and to permit reproduction of data recorded on the magneto-optical disc and the compact disc with good coupling efficiency and S/N ratio.